1. Field of the Invention
The present invention relates to a two-dimensional surface-emitting laser array. In particular, the present invention relates to a two-dimensional surface-emitting laser array that is used as a multibeam light source for an electrophotographic type image forming apparatus.
2. Description of the Related Art
A multibeam scanning device that is used for an electrophotographic type image forming apparatus condenses multiple beams from a multibeam light source as multiple light spots onto a surface to be scanned as follows.
More specifically, multiple beams from a multibeam light source having multiple light emission points are deflected simultaneously by a common light deflector (e.g., a polygon mirror), and the deflected multiple beams are condensed by a common scanning optical system onto a surface to be scanned as multiple light spots that are separated from each other in a sub scanning direction. Then, each of the light sources is driven according to an image signal so that multiple beams are scanned at one time for generating a two-dimensional image pattern.
Since this multibeam scanning device scans x (≧2) beams simultaneously, it can scan the beams at a speed x times a speed of a single beam scanning device if the scanning speed of the light spot is the same as that of the single beam scanning device. In other words, it is possible to speed up image processing.
In addition, if a scan line interval on a surface to be scanned is made to be 1/x times that of a single beam scanning device, it is possible to obtain an image having a resolution x times higher than that of the single beam scanning device in the scanning line direction.
In this way, the use of the multibeam as a light source in an electrophotographic type image forming apparatus is very effective in that an image can be formed with high resolution at high speed. As the number of the beams is increased, the effect thereof becomes larger.
Conventionally, a two-dimensional surface-emitting laser array disclosed in Japanese Patent Application Laid-Open No. 2001-350111 is known as one of the multibeam light sources of the electrophotographic type image forming apparatus described above.
Here, an arrangement of laser elements will be described in respect to the two-dimensional surface-emitting laser array that is used for the electrophotographic apparatus of the conventional example.
FIG. 4 is a diagram for illustrating an arrangement of laser elements of the two-dimensional surface-emitting laser array.
In the following description of a method of forming an image, it is a precondition that the surface to be scanned moves in a direction substantially perpendicular to the scan line, and that the direction of the scan line is referred to as a “main scanning direction” while the direction substantially perpendicular to the main scanning direction within the surface to be scanned, i.e., a feeding direction of the surface to be scanned is referred to as a “sub scanning direction”.
FIG. 4 illustrates a two-dimensional surface-emitting laser array 400, and a surface-emitting laser 410.
Then, a two-dimensional pattern of the light emission spots (one spot corresponds to one surface-emitting laser element) of the surface-emitting lasers 410 is defined by a base line U2 in the main scanning direction and a base line U1 in the sub scanning direction.
The light emission spots on in the first column are m light emission spots arranged linearly on the base line U1 with a first interval P1.
In addition, the light emission spots in the (k+1)th column are arranged with a second interval P2 from the light emission spots on the k th column (1≦k<n) along a base line A in a direction that is not perpendicular to the base line U1. In this way, the light emission spots are arranged until the n th column.
Here, the angle formed between the base line A and the main scanning direction (base line U2) is denoted by θ, and the component in the direction perpendicular to the main scanning direction of the interval between adjacent light emission spots on the base line A is denoted by P0. Then, it follows that “P0=P2×sin θ”, and P2 is determined so as to satisfy “P0×n=P1”.
As a result, m×n light emission spots are arranged one by one on m×n adjacent base lines U2 with the interval P0.
Note that the interval D between the columns in the main scanning direction is “D=P2×cos θ”.
Hereinafter, as indicators of the m×n light emission spots for description, expressions of one row and one column to m rows and n columns are used as illustrated in FIG. 4. For instance, a two-dimensional array of three rows and eight columns is illustrated in FIG. 4.
Multiple beams emitted from the two-dimensional surface-emitting laser array of the m rows and n columns formed as described above are condensed and scanned on a photosensitive member by a common scanning optical system (the lateral magnification in the sub scanning direction is set to Q times), so that m×n scan lines with an interval of Q×P0 are obtained.
Recently, there is an increasing demand for compact size, high resolution, and high speed of such a two-dimensional surface-emitting laser array that is used as a multibeam light source of an image forming apparatus.
In order to meet such request, it is necessary to arrange many elements in a small array area.
In other words, it is necessary to decrease the array area for realizing a compact size of the two-dimensional surface-emitting laser array. In addition, it is necessary to decrease the scan line interval for realizing high resolution of the two-dimensional surface-emitting laser array, and to increase the number of elements for realizing high speed of the two-dimensional surface-emitting laser array.
However, if the number of elements is increased, such a situation will occur that multiple electrical wirings for element drive should be arranged in some array interstices.
Therefore, if the array area is decreased, the array interstices are also narrowed so that it is difficult to arrange multiple electrical wirings.
Accordingly, it is necessary to secure intervals for arranging the multiple electrical wirings in the array interstices. However, if all the array interstices are of the same interval like the conventional structure, the interval sufficient for one single electrical wiring is the same as that for the multiple electrical wirings. Therefore, in such case there is a limitation in decreasing the array area.
Next, it will be described in more. specifically that the array area should be decreased for realizing a compact two-dimensional surface-emitting laser array as described above. As means for realizing a compact size thereof, a method of downsizing a casing of the device is mentioned.
As a method of downsizing a casing of the device, there is a method of shortening the optical path length of the scanning optical system. In order to realize this, it is possible to use a method of increasing the magnification of the scanning optical system.
However, if the magnification is increased, the scan line interval for the device necessary for obtaining the same scan line interval (i.e., same resolution) on a surface to be scanned is decreased according to the magnification thereof.
This means that the elements on the device become dense in the sub scanning direction (i.e., P0 becomes small).
If the P0 becomes small, P1 also becomes small. If P1 becomes smaller than the size of the surface-emitting laser element (desirably f20 microns or larger for obtaining relatively large emission intensity and good heat dissipation), the surface-emitting laser elements cannot be arranged.
In this case, the element interval can be secured in the sub scanning direction by increasing the number of elements in a direction other than the sub scanning direction in a two-dimensional grid array of the device. In other words, since “P1 (element interval on the same column)=n (number of elements belonging to the same row)×P0 (element interval in the sub scanning direction)” is a necessary condition for the m×n scan lines to have the equal intervals, it is sufficient to increase n for increasing P1.
However, in order to conduct image formation of light from the two-dimensional surface-emitting laser array on a surface to be scanned with sufficient position accuracy, it is necessary to use a part with an aberration within a permissible range in the scanning optical system.
More specifically, it is necessary to use a center of an optical element such as a lens as much as possible.
This means that a region where the light emission spots are located is required to be controlled to be of a certain area or smaller in the two-dimensional surface-emitting laser array.
In other words, there is a limitation in extending the array size in the main scanning direction (D×(n−1)) even in the case where n is increased.
For those reasons, it is necessary to decrease the array area for realizing a compact size of the two-dimensional surface-emitting laser array.
Next, it will be described in more specifically that if the number of elements is increased, multiple electrical wirings for element drive should be arranged in the array interstices, and that if the array area is decreased, it should be difficult to arrange the multiple electrical wirings as described above.
FIG. 5 illustrates a diagram for describing the difficulty in arranging the multiple electrical wirings as described above. In FIG. 5, the electrical wirings and pad electrodes are illustrated only partially for simple illustration.
The laser elements of a two-dimensional surface-emitting laser array that is used for an electrophotographic type image forming apparatus are supplied with current via individual electrical wirings and are driven individually.
More specifically, individual laser elements 420 of a two-dimensional surface-emitting laser array 401 are connected to corresponding pad electrodes 440 located in the peripheral portion of the array via individual electrical wirings 430 as illustrated in FIG. 5.
Here, if the two-dimensional surface-emitting laser array is made up of multiple elements, the number of electrical wirings for individually driving the elements is also increased corresponding to the number of the elements. Since the pad electrodes are located in the peripheral portion of the array, multiple electrical wirings for element drive should be arranged in some array interstices in some cases if the number of elements is increased.
In FIG. 5, for instance, multiple electrical wirings should be arranged in the interstices of a region 450.
For instance, if “((m−4)(n−4)>8” in a two-dimensional surface-emitting laser array that is used by individually driving m row and n column arrays (m≧5, n≧5), there is always an array interstice through which two electrical wirings pass.
This is because that since there are only (2m+2n−4) laser elements on the outermost peripheral portion of the array while there are (m−2)(n−2) elements inside thereof, if “(m−4)(n−4)>8”, the number of the inside elements will exceed the number of the outside elements.
The electrical wiring is usually made of a metal. If the width of the electrical wiring is small, the electrical wiring will be broken easily by electromigration.
Therefore, the electrical wiring for driving the surface-emitting laser that is usually driven by current of a few milliamperes is required to have a width of a few microns, for instance.
In addition, if the distance between electrical wirings is too small, crosstalk between electrical wirings may occur, which causes a serious influence on the image forming. Therefore, it is difficult to decrease the distance between elements in which multiple electrical wirings are arranged.
For those reasons, it is difficult to dispose many elements in a multi-element array having a small area because of the limitation of the electrical wirings.